informer

noun

Etymology: Inherited from Middle English enformour, from Anglo-Norman enfourmour; equivalent to inform + -er.

1. One who informs someone else about something.
2. A person who tells authorities about improper or illegal activity.

   “Being an informer often meant living in fear.”

   “The police relied on an informer to catch the gang.”

3. One who informs, animates, or inspires.

   “1729, Alexander Pope, Prologue to Sophonisba (by James Thomson Nature, informer of the poet's art.”